The field of the invention generally relates to an apparatus for providing an interface between a central office and a telephone subscriber, and more particularly relates to reducing the power requirements for an interface circuit that provides regulated current to a telephone subscriber.
Telephone subscribers typically have a voice signal and DC power fed to them from the central office through a loop interface circuit onto a pair of wires connected to the subscriber telephone. Conductive or resistive power and the voice signal may be applied to the loop interface circuit either through a transformer or capacitor. It is becoming increasingly common to insert electronic systems between the central office and the subscriber telephone either to increase the voice travel on each pair of wires, to add or increase the data handling capability of a pair of wires, or to improve the quality of service to the subscriber in some way. Most of these electronic systems provide all the services to the subscriber telephone that the central office normally provides. These services include providing the following to the telephone subscriber: regulated direct current, voice signals, off-hook and dialing signals (either pulse or tone), and ring signals.
Providing regulated direct current to the telephone subscriber requires components between the power source and the subscriber telephone These components disrupt power and reduce the efficiency of the system. Direct current must have a minimum of 20 milliamps, and in some cases, 25 milliamps. The central office current source typically includes a battery supplying 48 volts direct current, though less voltage would be acceptable subject to the minimum current requirements.
A few methods have traditionally been used to supply direct current and the voice signal to the telephone subscriber. In a first method, a transformer is used to couple the voice signal onto a telephone line having a pair of wires connected to the telephone subscriber. In this method, a power supply having a fixed constant voltage output is coupled to the transformer and supplies direct current through the pair of wires to the telephone subscriber via this same transformer. A drawback of this method is that transformers can saturate when a large amount of current flows through transformer windings. This saturation results in distortion of the voice signal being provided to the telephone subscriber. Consequently, supplying direct current through the transformer winding requires a relatively large magnetic core to prevent the magnetic core from becoming saturated.
A further drawback of this method is that the direct current resistance of the telephone line varies with the length of the telephone line. Consequently, a power supply with a constant voltage output sufficient to supply the minimum current on a long telephone line would supply considerably higher current than necessary on a short line. Thus, to prevent the core of the transformer from saturating, the transformer would have to be two to four times larger to handle the current of the short line than if the transformer and supply were dedicated for use only on a long line. Another drawback of this method is that a larger power supply may be necessary to meet the current requirements when providing power with a short telephone line.
An improvement on this first supply method is using a power supply having a fixed constant current output instead of a fixed constant voltage output. Using a power supply with a constant current output allows a two to four times reduction in the transformer size. With a constant current output, the current through the transformer is fixed and the size of the transformer is selected accordingly. With a constant voltage output, the transformer size must be selected for worst case condition that prevents the transformer core from saturating. However, there is still a significant power loss to having a constant current output, especially on communications systems having a telephone line with a short length. In short length telephone line systems, most of the system supply voltage is dropped across the current source.
A second method of supplying DC power to the telephone subscriber has generally been used by the medium to larger electronic systems. In large systems, the total power requirements are too large to be supplied over the telephone line between the central office and the subscriber via the loop interface circuit. In these systems, commercial AC power is provided on the subscriber end to directly power battery chargers and power supplies. These batteries and power supplies directly feed power via line cards and telephone lines to the telephone subscribers. In order to reduce the physical size of the line cards, the direct current is not supplied through a voice frequency transformer on the line card, but is coupled to the telephone subscriber through two resistors, typically having about 450 ohms each. One resistor is coupled from the positive side of the power supply to the tip side of the telephone line, and the other resistor is coupled from the negative side of the power supply to the ring side of the telephone line. Coupling the power supply in series with the resistors to both the tip and ring lines balances the telephone line and rejects longitudinally induced AC currents, most notably 60 hertz currents induced from power lines extending adjacent the telephone subscriber lines. The voice signal is capacitively coupled to the telephone line to eliminate the requirement of having direct current being fed to the telephone subscriber through the transformer.
This second method reduces the transformer size requirement compared to the first method but increases the system power requirements because of the power lost due to heat generated when current is fed through the DC resistors. Further, most of the heat dissipated is due to the current fed through the DC resistors as the transformer resistance is generally less than 100 ohms. Further, as systems get larger and provide power to more subscriber lines, this heat becomes a problem. Another problem with this method is that larger batteries and chargers must be used, as the power supply must be sized for worst-case applications, i.e. when all lines short circuit.
Another method of supplying DC power is with a current source connected directly to the telephone line with the voice signal on it. This method requires two current sources: one coupled to the tip line and one coupled to the ring line to maintain longitudinal balance. When two current sources are used in this way, each must supply the identical current to keep the other current source from saturating. When a current source saturates, it will essentially short one side of the telephone line. Most likely, a feedback method would have to be employed with this method to maintain the line voltage centered between the power supply voltage. This method has a drawback of requiring elaborate circuitry to protect the solid state devices that would be used to implement this feedback because these devices are connected directly to the subscriber telephone line and are susceptible to transient signals on this line, i.e. lightning.
In the last few years, integrated circuit manufacturers have offered line interface integrated circuits which have two current sources and the necessary balance feedback. One current source is coupled to the tip line and the other current source is coupled to the ring line. However, these integrated circuits have the drawback of requiring a large voltage drop across each of the current sources to prevent saturation caused by peak voice signal levels on tip and ring. Saturation of the current source may result in the voice signals being clipped. Further, requiring a large voltage drop requires a larger power supply.
These integrated circuits also have the drawback of requiring circuitry to provide transient protection, as these integrated circuits can withstand peak voltages only on the order of 70 volts. Consequently, these integrated circuits may not be practical in systems where the subscriber telephone line is exposed to lightning and AC power system transients.